This invention relates to a television system and, especially, to such a system in that the high frequency components of luminance and/or chrominance signals are multiplexed with a television signal of the standard system for transmission, thereby obtaining an image improved in resolution of brightness and/or color in the receiver.
While, in the present television system, the number of scanning lines is 525 (effectively, about 480 lines) and the horizontal bandwidth is 4.2 MHz, the transmitting equipments have a power which enables such a high resolution treatment corresponding to 1.5 to 2 times horizontal resolution and 6 to 8 MHz horizontal bandwidth. Due to limitation of the bandwidth of the broadcasting wave, it is impossible to transmit all of the high resolution image informations existing in the transmitter side to the receiver side. However, if the high resolution image information is transmitted in respect of a part of the image screen, such as the central part or the title part, in which high resolution is desired, the resultant reproduced image can provide the viewer with such an impression in that the whole image has been improved in resolution.
An improved system which was developed from this point of view is disclosed in the Japanese opened patent gazette No. S62-245786. In this system, the image screen is finely divided into small blocks and each block is provided with an address. Part of pixels are thinned out from each block and a television signal is prepared mainly from a luminance signal based upon the remaining pixels. For such a block which requires high resolution, a difference signal between the mean value of the luminance signal of the block and luminance information of the pixels thinned out is multiplexed to the television signal together with the address of the block for transmission.
In reception, the multiplexed signals are separated and the luminance signal of the pixels which were thinned out in the transmitter side is recovered from the mean value of the luminance signal of the block specified by the address and the above-mentioned difference signal. The recovered luminance information is inserted between the luminance signals to recover the original form before thinning in the transmitter side and to display the same. Accordingly, in the displayed image, high resolution is obtained only in those blocks having the difference signal transmitted, while the remaining portion exhibits resolution similar to that of the prior art television display screen.
When the above-mentioned television signal is received by a prior art receiver, compatibility of the receiver is obtained so far as it goes, since the whole image is displayed at resolution similar to that in the case of reception of the prior art television broadcast.
In the above-cited system, however, the amount of high resolution information which can be multiplexed to the television signal is limited and, therefore, it may be impossible to transmit the thinned information, in spite of a considerable amount of high resolution information included in the original signal in a certain block. This may result in some frequency turnaround at the position of that block in the screen, due to lack of high resolution information.
Moreover, in the case of receiving a broadcasting wave of the above-cited system by a conventional receiver, a little discontinuity of frequency characteristic occurs between the blocks since the pixels are treated for each block, and periodic non-uniformity is sometimes noticeable in some kind of image pattern since the blocks are arranged regularly.
Furthermore, the amount of high resolution information which can be transmitted is reduced by the amount corresponding to the address signal, since the address signal must be transmitted in combination with the high resolution information.